Atrial fibrillation (AF) remains the most commonly occurring cardiac arrhythmia. It is associated with a lower quality of life and a higher rate of morbidity & mortality primarily due to poor hemodynamics and often stroke. One of the main options to treat atrial fibrillation is to ablate abnormal sources of electrical activity using percutaneous radiofrequency (RF) catheters. RF lesions isolate the spread of abnormal electrical activity from various sites in the heart including the pulmonary veins. As a result, a cornerstone of therapy in AF ablation procedures is to electrically isolate the pulmonary veins (PVs) with RF lesions, essentially creating an electrical barrier between the arrhythmogenic foci inside the PVs and the rest of the heart. Often the cardiologist over- ablates or under-ablates leaving gaps which can still transmit the abnormal electrical activity to the rest of the heart. To date, there are limited means for real-time monitoring of tissue injury during RF ablation procedures and there are no means of directly visualizing ablated cardiac tissue to distinguish it from viable conducting tissue. Here we propose to create a new generation of imaging catheters to distinguish normal cardiac tissue from ablated cardiac tissue in real time. They operate based on spectral changes in tissue autofluorescence caused by thermal damage. Specifically, in Phase II we will expand the design of our current, 1st generation catheter, that relied on acquiring emission from a single spectral band, to include acquisition from multiple emission bands within an autofluorescence range of 380-600nm and to use post-acquisition analysis to reveal thermal damage to layers of endocardial collagen and underlying atrial muscle. The UV multispectral catheter will be then used to examine and visualize percutaneous RF ablation lesions in live large animals such as pigs. We also test on freshly excised donated human atrial tissue. The catheter will undergo extensive documentation, verification, and validation in order to be approved for use in a human study. Manufacturing process instructions will be developed and reviewed by the manufacturing partners who will construct, package, and sterilize the various catheter components. We will work with our clinical partners to develop a human study protocol, and will develop informed consent documentation, instructions for use, and other documentation necessary for ethics review and FDA approval. At the end of Phase II we aim to have a fully functional, statistically tested 12 Fr multispectral imaging catheter, approved and ready for first-in-man studies. Our ultimate long term goal is to develop a new generation of ablation catheters enabling real-time visualization of ablated cardiac tissue. This technology will pave the way for easier, faster, safer, more cost- effective, more reliable and minimally invasive AF ablation procedures by allowing cardiologists to see in real time if lesions are complete or if there are gaps that need to be filled in. in the case of a patient who has a recurrence post-ablation, the catheter will be used to quickly and efficiently identify where the gap is if one exists.